1. Field of the Invention
The present invention relates to a power steering apparatus and a preload adjustment method thereof.
2. Description of Related Art
In a motor-driven power steering apparatus, a rotation force of a motor for assistance generating an assist force is generally transmitted through a speed reduction mechanism to a pinion shaft and so on connected to a steering shaft through a torsion bar. The speed reduction mechanism is formed by, for example, a worm gear mechanism. In the case where the motor-driven power steering apparatus is a pinion-assist type apparatus, the speed reduction mechanism is formed by including a worm connected to an output shaft of a motor and a worm wheel fixed to the pinion shaft and engaged with the worm.
In the above motor-driven power steering apparatus, it is necessary to set a center distance between the worm and the worm wheel easily without being affected by dimension errors of parts such as the worm at the time of assembly as well as to maintain the center distance suitably to thereby eliminate backlash even when the engagement between the worm and the worm wheel is changed over time after the assembly.
In response to the above problems, there is disclosed in JP-A-2013-208932 (Patent Document 1), a power steering apparatus having a preload means for biasing a bearing supporting a distal shaft portion of the worm to a given preload direction so as to apply the preload to an engaged portion of the worm and the worm wheel. The preload means is formed by a compression coil spring. The center distance between the worm and the worm wheel is suitably maintained by an elastic force of the preload means to prevent a backlash.
The preload means described in Patent Document 1 includes a bearing case which is a C-shaped annular body in which part in a circumferential direction is cut out. An inner circumferential surface of the bearing case is formed as a bearing housing hole in which a guide surface for guiding the bearing to the preload direction is formed. One end of a compression coil spring abuts on the bearing through a spring insertion hole formed in the bearing case and the other thereof abuts on a lid fixed on a lid attachment surface of a housing. The lid is fixed to the lid attachment surface of the housing by a bolt.
A biasing force of the compression coil spring depends on a compression amount of the spring. The compression amount depends on a distance between a position of the worm and the lid attachment surface, a free length of the spring and the like. However, variation may occur in the distance between the position of the worm and the lid attachment surface, the free length of the spring and the like due to manufacturing errors, assembly errors and so on. Therefore, variation may occur in the biasing force of the compression coil spring and variation may occur in the preload between the worm and the worm wheel due to the above variation.